Condensation apparatus

ABSTRACT

The invention provides an apparatus for increasing the size of gas-entrained particles in order to render the gas-entrained particles detectable by a particle detector, the apparatus comprising an evaporation chamber ( 2 ) and a condenser ( 7 ); the apparatus is configured so that vapour-laden gas from the evaporation chamber can flow into the condenser and condensation of the vaporisable substance onto gas-entrained particles in the condenser takes place to increase the size of the particles so that they are capable of being detected by a particle detector.

CROSS REFERENCE TO RELATED APPLICATION

This application is the National Phase filing of InternationalApplication No. PCT/GB2009/001147, filed May 8, 2009, which claimspriority to GB Application No. 0808385.9, filed May 8, 2008. The entirecontent of each prior application is hereby incorporated by reference.

This invention relates to a condensation apparatus for use with particlecounters. More particularly, the invention relates to a condensationapparatus that can increase the effective size of gas-entrainednano-particles so that they can be detected by an optical particlecounter.

BACKGROUND OF THE INVENTION

There is currently a great deal of concern about the health effects ofnano-particles emitted unintentionally into the air. For example, the500% increase in respiratory illness and allergies in the UK in recentyears has been associated in part with particles emitted by dieselengines and other combustion processes. Whilst the main focus has beenon diesel emissions, attention is turning to other potential sourcessuch as power generation using fossil fuels, incineration, nuclear powergeneration and aircraft emissions. All heavy industries involvingprocesses emitting fumes have potential problems with the emission ofnano-particles. Such processes include smelting, firing, glassmanufacture, welding, soldering, nuclear power generation andincineration. There is also concern amongst consumer companies thatenzymes in washing powders, powder coatings and fibres used indisposable nappies and other products could cause problems. In addition,the US EPA is becoming increasingly concerned about gasoline engineemissions.

Nano-particles are known to produce toxic effects. For example, they cancross the blood-brain barrier in humans and gold nano-particles can moveacross the placenta from mother to foetus. Early studies with PTFE(polytetrafluoroethylene) particles around 20 nm in diameter showed thatairborne concentrations of a supposedly inert insoluble material lowerthan 50 μg/m³ could be fatal to rats. Moreover, nano-tubes produce amore toxic response in rats than quartz dust.

In addition to concerns from a health perspective, the elimination orcontrol of airborne nano-particles is important in maintaining standardsin the many thousands of clean rooms in the micro-electronics,pharmaceutical, medical, laser, and fibre optics industries.

Small particles can be classified as shown in Table 1 below.

TABLE 1 Term Aerodynamic Particle Size Range Dust D > 10 μm Coarseparticles 2.5 μm < D < 10 μm Fine particles 0.1 μm < D < 2.5 μmNano-particles or ultrafine particles 1 nm < D < 0.1 μm

The term “nano-particles” is used to refer to particles having anaerodynamic particle size in the range from 1 nm to 0.1 μm (100 nm).

For spherical particles, the aerodynamic particle size is the diameterof the particle. Real particles in the air often have complicatedshapes. For non-spherical particles, the term “diameter” is not strictlyapplicable. For example, a flake or a fibre has different dimensions indifferent directions. Particles of identical shape can be composed ofdifferent chemical substances and have different densities. Thedifferences in shape and density cause considerable confusion indefining particle size.

The terms “aerodynamic particle size” or “aerodynamic diameter” aretherefore used in order to provide a single parameter for describingreal non-spherical particles having arbitrary shapes and densities. Asused herein, the term “aerodynamic diameter” is the diameter of aspherical particle having a density of 1 g/cm³ that has the sameinertial property (terminal settling velocity) in the air (at standardtemperature and pressure) as the particle of interest. Inertial samplinginstruments such as cascade impactors enable the aerodynamic diameter tobe determined. The term “aerodynamic diameter” is convenient for allparticles including clusters and aggregates of any forms and density.However, it is not a true geometric size because non-spherical particlesusually have a lower terminal settling velocity than sphericalparticles. Another convenient equivalent diameter is the diffusiondiameter or thermodynamic diameter which is defined as a sphere of 1g/cm³ density that has the same diffusivity as a particle of interest.

The investigation and monitoring of nano-particles in the atmosphere hasbeen hampered by a shortage of instruments which can measure in thenano-particle range but which are sufficiently inexpensive, robust andconvenient to be used on a widespread basis.

Some instruments for measuring nano-particles are known which make useof laser optics to detect and measure particles. However, becauseoptical measurements cannot readily be used to detect particles in thenano-particle size range, techniques have been developed for “growing”particles to make them larger and therefore detectable and thistechnique forms the basis for Condensation Particle Counters.Condensation Particle Counters (CPCs) work by passing a sample ofairborne particles through a chamber containing a vapourised liquid andthen through a condenser where the vapourised liquid is condensed ontothe airborne particles to form droplets of a size that can be measured.One example of such an instrument is disclosed in WO 02/029382 (Ahn etal). The CPC disclosed in WO 02/029382 comprises a cylindricalevaporation chamber which is lined with a porous absorbent supportformed from a material such as nonwoven fabric. At one end of thechamber, the porous absorbent support is in contact with a reservoir ofa volatile liquid such as isobutanol so that the liquid can travel alongand soak the support by capillary action. The exterior surface of theevaporation chamber is surrounded by a heating element that heats thechamber causing isobutanol to evaporate from the support thereby tocreate a vapour-filled chamber. Air samples suspected of containingairborne particles are introduced into the chamber at the reservoir endand drawn through the chamber into a condenser where the condensation ofthe isobutanol vapour onto the airborne particles takes place to formdroplets that can be measured using an optical particle counter.

An example of a commercially available CPC making use of the principlesdisclosed in WO 02/029382 is the Model 3025A Ultrafine CondensationParticle Counter available from TSI Incorporated, Shoreview, Minn.,U.S.A.

Another known apparatus is the handheld CPC 3007 from TSI (www.tsi.com),and the operation of this is described in more detail below in relationto FIG. 1.

Existing Condensation Particle Counters suffer from a number ofdisadvantages. For example, they tend to require a high powerconsumption in order to heat the working fluid and have a long (10 to 20minutes) warming up time before they can be used. These disadvantagesarise at least in part because the evaporation chamber is heated bymeans of an external heating element and therefore the entire casingsurrounding the chamber must heated before the instrument reaches theoperating temperature. Furthermore, with known CPCs, there is arelatively high consumption of the working fluid (e.g. isobutanol) withthe result that the working fluid must be topped up on a frequent basis,often before each use. Even in the case of the TSI US 3007 handheldcondensation particle counter, the working fluid cartridge with theworking fluid must be replaced on a regular basis. A furtherdisadvantage of known CPCs is the unpleasant smell of the working fluidsused (e.g. iso-butanol) and the relatively high costs.

At present, therefore, there remains a need for a Condensation ParticleCounter that can be used for long periods without topping up the workingfluid, which has a greatly reduced warm-up time and which lends itselfto miniaturisation.

SUMMARY OF THE INVENTION

The present invention sets out to provide a condensation apparatuswhich, when used in a Condensation Particle Counter, can overcome or atleast alleviate some or all of the problems described above in relationto known CPCs.

In a first aspect, the invention provides apparatus for increasing thesize of gas-entrained particles in order to render the gas-entrainedparticles detectable by a particle detector, the apparatus comprising anevaporation chamber and a condenser;

-   -   the evaporation chamber having an inlet for admitting gas into        the evaporation chamber and an outlet through which vapour-laden        gas may leave the evaporation chamber;    -   the evaporation chamber having disposed therein a heating        element and a porous support, the heating element being in        direct contact with the porous support, wherein the porous        support carries thereon a vaporisable substance and the heating        element is heatable to vaporise the vaporisable substance to        form vapour within the evaporation chamber;    -   the condenser being in fluid communication with the outlet of        the evaporation chamber, and the condenser having an outlet for        connection to the particle detector.    -   the apparatus being configured so that vapour-laden gas from the        evaporation chamber can flow into the condenser and condensation        of the vaporisable substance onto gas-entrained particles in the        condenser takes place to increase the size of the particles so        that they are capable of being detected by a single particle        detector.

In one embodiment, the evaporation chamber has at least two inlets, oneof which serves to admit a sample gas containing gas-entrained particlesinto the evaporation chamber and another of which is connectable to asource of substantially particle-free carrier gas.

In another embodiment, the condenser has at least two inlets, one inletbeing in fluid communication with the evaporation chamber and anotherinlet serving to admit a sample gas containing gas-entrained particlesinto the condenser.

In a second aspect of the invention, there is provided apparatus forincreasing the size of gas-entrained particles in order to render thegas-entrained particles detectable by a particle detector, the apparatuscomprising:

-   -   a source of a vaporisable substance;    -   heating means to bring about evaporation of the vaporisable        substance to form vapour;    -   an inlet for admitting a sample gas containing gas-entrained        particles;    -   a condenser, the condenser being provided with an outlet for        connection to the particle detector;        the apparatus being configured such that condensation of the        vapour onto gas-entrained particles takes place in the condenser        to increase the size of the particles so that they are capable        of being detected by the particle detector;    -   characterised in that the vaporisable substance is selected from        dimethyl phthalate, dioctyl phthalate and dimethylsulphoxide.

In a further aspect, the invention provides a condensation apparatus forincreasing the size of gas-entrained particles in order to render thegas-entrained particles detectable by a particle detector, the apparatuscomprising:

-   -   an evaporation chamber;    -   a condenser in fluid communication with the evaporation chamber        and having an outlet for connection to a particle detector;    -   a heating element and a porous support each disposed within the        evaporation chamber, the porous support carrying thereon a        vaporisable substance and the heating element being heatable to        vaporise the vaporisable substance to form vapour within the        evaporation chamber;    -   a first inlet for admitting a stream of carrier gas into the        evaporation chamber to carry vapour through to the condenser;    -   a second inlet which is downstream of the porous support and        through which a stream of sample gas containing gas-entrained        particle can be introduced;    -   the apparatus being configured so that condensation of the        vaporisable substance onto the gas-entrained particles in the        sample gas takes place in the condenser to increase the size of        the particles so that they are capable of being detected by a        particle detector.

As described above in the introductory section of this application, manyparticle counters, particularly those based on optical methods ofparticle detection, are unable efficiently to detect and count particleshaving a particle diameter of less than about 300 nm. The condensationapparatus of the invention enables particles of much smaller size (e.g.an aerodynamic particle diameter down to less than 3 nm) to be detectedand achieves this by growing the particles by condensing onto them avaporisable condensable substance.

The vaporisable substance can be a liquid or a vaporisable solid. Wherethe vaporisable substance is a solid at room temperature, it ispreferably one that melts first to form a liquid and then forms a vapourfrom the liquid state rather than a substance that sublimes from thesolid state.

Examples of solid materials that can be used as the vaporisablesubstance include solid hydrocarbons and long chain carboxylic acids,e.g. fatty acids such as stearic acid.

It is currently preferred, however, that the vaporisable substance is aliquid.

Liquids that may be used include water and alcohols such as propanol,isopropanol and isobutanol, or higher boiling organic liquids. Asdiscussed above in the introduction, one of the disadvantages of knowncondensation particle counters is that liquid used as the vaporisablesubstance is consumed within a relatively short period of time andtherefore fresh liquid must be added at frequent intervals. With someknown condensation particle counters, it is necessary to add more liquideach time the apparatus is used.

In order to overcome the disadvantages associated with knowncondensation particle counters, it is preferred to use as thevaporisable substance a liquid having a boiling point at atmosphericpressure of at least 110° C.

One group of preferred vaporisable liquids consists of dimethylphthalate, dioctyl phthalate and dimethylsulphoxide. One particularlypreferred liquid is dimethyl phthalate. By using higher boiling liquidssuch as dimethyl phthalate, the rate of consumption of the liquid isgreatly reduced and hence the liquid does not need to be topped up sofrequently.

Where the vaporisable substance is a liquid and the evaporation chambercontains or is linked to a reservoir of liquid, there is a possibilitythat tipping the apparatus (e.g. while in transit) could cause liquid toleak into any inlets or outlets of the evaporation chamber. In order toprevent or minimise the likelihood of this occurring, the inlet(s) andoutlet(s) of the evaporator chamber can be provided with a lip or rimwhich acts as a barrier to liquid. It will be appreciated that theheight of the rim or lip will depend upon the volume of liquid carriedin the reservoir. The lip or rim may be defined or provided by the endof an inlet or outlet tube extending into the evaporation chamber. Byway of example, the rim or lip may be from 1 to 8 mm high, morepreferably 2 to 5 mm high.

The carrier gas may be air or a pure gas or mixture of gases. Forexample, instead of air, the carrier gas could be nitrogen gas. Thecarrier gas is preferably filtered so that particles and otherimpurities are not carried through the evaporation chamber into thecondenser. The carrier gas can be provided from a particle-free source,for example a cylinder of gas. Alternatively or additionally, a filtercan be located externally of the evaporation chamber. For example, afilter can be located across the first inlet itself, or a filter can belocated upstream of the first inlet, so that, in either case, carriergas entering the evaporation chamber is free from impurities andespecially particulate impurities. Examples of filters include HEPAfilters and such filters are well known and do not need to be describedin detail here.

In an alternative arrangement, the carrier gas can be filtered after ithas entered the evaporation chamber. For example, in one embodiment, afilter can consist of or comprise the porous support for the vaporisablesubstance. In this embodiment, the porous support acts as a filtermembrane that extends across the interior of the evaporation chamberdividing it into an upstream compartment and a downstream compartment.Carrier gas entering the upstream compartment through the first inletmay contain particulate impurities which are removed as the carrier gaspasses through the porous support, whilst at the same time thevaporisable substance on the porous support is evaporated and the vapouris carried off by the filtered carrier gas. Thus, on the downstream sideof the porous support, there are no particulate impurities present. Itwill be appreciated that by “particulate impurities” is meant particlesother than those that are intended to be detected and counted.

The porous support can take various forms and be made from any of anumber of different materials. For example, the porous support can beformed from a porous ceramic material or a porous fabric such as glasscloth, quartz fibre filter rockwool or a cotton fabric. The porousmaterial should be stable at the temperatures used to vaporise thevaporisable substance and, when the vaporisable substance is a liquid,should preferably be wettable by the substance.

A temperature sensor e.g. a thermocouple) is typically disposed withinthe evaporation chamber for sensing the temperature inside the chamberinterior. The temperature sensor is preferably in thermal contact withor in close proximity to the heating element. The temperature sensor maybe arranged so that it is encircled by the heating element and/or theporous support. The temperature sensor is typically connected to atemperature control device.

The heating element can take various forms but, in each case, theheating element is disposed inside the evaporation chamber and is inclose proximity to the porous support rather than surrounding theexterior of the chamber (as is the case in known commercially availablecondensation particle counters). A significant advantage in placing theheating element inside the evaporation chamber is that it greatlyreduces the warm-up time of the instrument and the power consumption ofthe instrument. Thus, CPCs containing the condensation chambers of theinvention can be warmed up to operating temperature in under a minute incontrast to the 10-20 minutes required for known CPCs to reach operatingtemperatures.

Most preferably the heating element is in direct contact with the poroussupport.

For example, the porous support can surround the heating element.

In one embodiment, the heating element comprises a rod (e.g. cylindricalrod) portion and the porous support surrounds the said rod (e.g.cylindrical rod) portion. For example, the porous support can comprise asleeve that fits over the rod (e.g. cylindrical rod) portion of theheating element. Such a form of construction is particularly suitablefor use when the porous support is formed from a porous fabric ashereinbefore defined.

The porous support (e.g. sleeve) may have a downwardly depending portionwhich, in use, extends into a reservoir of the vaporisable substance(when a liquid).

The rod portion of the heating element can have a hollow interior withinwhich is disposed a heater wire or heater probe and optionally athermocouple. In order to ensure good thermal contact between the heaterwire or heater probe and the inner surface of the hollow rod, athermally conductive filler may be used to hold the heater wire orheater probe and the thermocouple (when present) in place. Examples ofthermally conductive fillers include solders and other low meltingalloys, and thermally conducting resins such as metal particle-filledresins (e.g. epoxy resins).

Vaporising devices incorporating heating elements of the aforesaid typeare believed to be new and represent a further aspect of the invention.Accordingly, in another aspect, there is provided a vaporiser device foruse in a condensation particle counter, the vaporiser device comprising:

-   -   a heating element comprising a mounting portion for installing        in a wall of an evaporation chamber in the condensation particle        counter, and a rod portion; the rod portion being arranged to        extend inwardly into the evaporation chamber in use; and    -   a porous support which surrounds and is in contact with the rod        portion, the porous support carrying or being capable of        carrying a vaporisable substance;    -   and optionally retaining means for holding the porous support in        place on the rod portion.

The rod portion of the heating element and the porous support may be asdefined above.

The retaining means can comprise or consist of a clip or perforatedsleeve that fits over the porous support to hold it in place.

The evaporation chamber may vary in cross sectional shape and can be,for example, of circular or rectangular cross section.

The apparatus of the invention may be provided with a second inletthrough which is introduced a stream of sample gas containing thegas-entrained particles to be counted. The second inlet can be disposedso that it opens into the evaporation chamber, or into an intermediatechamber between the evaporation chamber and the condenser, or into thecondenser.

In one embodiment, the second inlet is arranged so that it opens intothe evaporation chamber. The second inlet may have a nozzle that extendsinto the evaporation chamber. When the second inlet is located in theevaporation chamber, it is preferably in-line with an exit openingcommunicating with the condenser, e.g. so that a longitudinal axis ofthe inlet is aligned with a longitudinal axis of the condenser. Thesecond inlet preferably has a cross sectional area less than the crosssectional area of the exit opening; e.g. the second inlet when circularhas a diameter less than the diameter (when circular) of the exitopening. In this embodiment, without wishing to be bound by any theory,it is believed that a stream or jet of the sample gas is surrounded by aconcentric layer of carrier gas and vapour as it leaves the evaporationchamber, mixing of the two concentric layers taking place as they movealong the condenser.

In another embodiment, the second inlet is arranged so that it opensinto the condenser. Preferably, an exit opening of the evaporationchamber is provided with a nozzle that extends into the condenser to aposition level with or downstream of the second inlet. With thisarrangement, without wishing to be bound by any theory, it is believedthat a stream of carrier gas and vapour from the evaporation chamber issurrounded by a concentric layer of sample gas as it enters thecondenser, mixing of the two concentric layers taking place as they movealong the condenser.

Where the second inlet opens into the condenser, the sample gas may bepartially or wholly saturated with vapour before it enters thecondenser. In this embodiment, the second inlet may be connected to anancillary evaporation chamber.

In a further embodiment, the apparatus is configured such that:

-   -   the second inlet is arranged so that it opens into an        intermediate chamber between the evaporation chamber and the        condenser;    -   the intermediate chamber is divided by a dividing wall into        upstream and downstream sub-chambers, a central hole in the wall        providing communication between the sub-chambers, whereby the        second inlet opens into the upstream sub-chamber;    -   a third inlet opens into the downstream sub-chamber, the third        inlet being connectable to a supply of filtered gas;    -   a nozzle is provided that extends from an exit opening of the        evaporation chamber into the condenser to a position in the        upstream sub-chamber that is level with or downstream of the        second inlet;    -   the downstream sub-chamber contains a cylindrical baffle that is        aligned with the said nozzle and the central hole in the        dividing wall, and the third nozzle opens into a space        surrounding the cylindrical baffle.

With the foregoing arrangement, without wishing to be bound by anytheory, it is believed that a stream of carrier gas and vapour from theevaporation chamber is surrounded by a concentric layer of sample gas asit exits the nozzle into the upstream intermediate sub-chamber. As thetwo concentric layers of carrier gas/vapour and sample gas pass throughthe central hole in the dividing wall into the downstream intermediatechamber, they are surrounded by a further concentric layer of filteredcarrier gas entering through the third inlet. Thus there is formed,temporarily, a tri-laminar stream of gas consisting of a central core ofcarrier gas and vapour, an intermediate layer of sample gas containinggas-entrained particles, and an outer layer of filtered carrier gas. Thetri-laminar stream of gas then exits the intermediate chamber throughthe interior of the cylindrical baffle and into the condenser wheremixing of the three layers occurs.

In each of the foregoing embodiments, as the mixture of heated carriergas, sample gas, gas-entrained particles and vapour passes along thecondenser, cooling leads to the gases within the condenser becomingsupersaturated with the vapour of the vaporisable substance with theresult that it condenses onto the surface of the particles. When thevaporisable substance is a liquid, droplets are formed on or around theparticles, whereas when the vaporisable substance is a normally a solidat room temperature, cooling leads to the formation of beads containingor bearing the particles. In this way, the size of the particles iseffectively increased from sizes as low as 3 nm to sizes up to and inexcess of 1 μm. By increasing the size of the particles, they arerendered detectable by optical particle detectors such as opticalparticle counters.

The condenser is typically formed from a material of high thermalconductivity and is made sufficiently long to ensure that the mixture ofcarrier gas, sample gas, vapour and particles to be detected coolssufficiently to allow the vaporisable substance to condense onto theparticles to grow the particles to a detectable size (e.g. 1 μm orgreater). The condenser can therefore take the form of a tube formedfrom a metal material such as aluminium or stainless steel. Thethickness of the walls of the condenser and other parts of the apparatuscan be as thin as practical, but, when there is an intention to use theapparatus under elevated or reduced pressure (e.g. at 10 bar), the wallshould be sufficiently thick to withstand such pressures.

In order to assist cooling of the mixture of gases, vapour and particlesin the condenser, cooling means may be provided.

In one embodiment, the cooling means comprises one or more fans eachdirecting a flow of air onto the external surface of the condenser. Inone embodiment there is one fan. In another embodiment there are twofans.

The fan(s) may or may not be part of an air temperature controllingsystem. The temperature controlling system enables the air cooling thecondenser surface to be maintained at a pre-set temperature which is notinfluenced by the temperature of the air surrounding the condensationchamber.

Alternatively, a cooling element may be located in contact with theexternal surface of the condenser. The cooling element can be, forexample, a thermoelectric cooling device (e.g. a Peltier coolingdevice).

In order to facilitate improved cooling, the cross-section of thecondenser may be formed in such a way as to enhance the ratio of thecircumference to cross-section area.

The condenser preferably has a surface area to volume ratio which isgreater than the surface area to volume ratio of a cylinder. Byincreasing the surface area to volume ratio, the condenser can be mademore efficient resulting in more rapid cooling and thereby enabling thesize of the condenser to be reduced.

A condenser can be defined as having a length (a dimension correspondingto the distance between the inlet and outlet of the condenser), a width(a dimension orthogonal to the length) and a height (a dimensionorthogonal to the length and height). In the case of a tubular condenserof circular cross section, the width and the height are the same andboth correspond to the diameter of the tube. In the case of arectangular condenser of square cross section, the width and height arealso the same. However, in this application, where the width and heightof a condenser are not the same, the reference to “height” means thelesser of the two dimensions.

The surface area to volume ratio of a condenser can be increased in anumber of ways. For example, at least part of the condenser may have aportion of flattened cross section or may be have an elongate oval orrectangular shape in cross section, i.e. a cross section in which theheight is substantially less than the width of the condenser. In onepreferred embodiment, the condenser is substantially rectangular incross section wherein the height is less than half the width.

In another embodiment, the condenser may comprise an annular or partannular condenser body. An annular condenser body may be formed from twoconcentric cylinders with the hot vapour laden gas being directedthrough the annular space between the inner and outer cylinders andcooling air being directed through the interior of the inner cylinder aswell as around or against the outer surface of the outer cylinder.

In order to enable particle sizes to be measured accurately, it isimportant to ensure that the residence time of each particle in thecondenser is substantially the same. This means that the flow velocitiesand flow paths of the particles through the condenser should ideally beas uniform as possible.

Where a non-cylindrical condenser (e.g. a rectangular condenser) isconnected to in-line cylindrical inlets and outlets, there exists thepossibility of non-uniform flow between the inlet and outlet,particularly in cases where the width (as defined herein) of thecondenser is greater than the diameters of the inlet and outlet. Inorder to overcome this potential problem, a condenser (e.g. asubstantially rectangular condenser) may be provided with a pair of flowdistributor tubes which are aligned substantially at right angles withrespect to the length (direction of flow) of the condenser. The flowdistributor tubes are connected to the inlet and outlet of the condenserand each extend across the width of the condenser and are provided withelongate slots or arrays of holes which open into the interior of thecondenser.

Accordingly, in another aspect of the invention, there is provided acondenser for use with an apparatus of the invention as defined herein,the condenser comprising:

-   -   a condenser body having an inlet, an outlet and a hollow        interior which has an internal length, an internal width and an        internal height;    -   an inlet flow distributor tube connected to the inlet of the        condenser body and extending across the internal width of the        condenser body; and    -   an outlet flow distributor tube connected to the outlet of the        condenser body and extending across the internal width of the        condenser body;    -   wherein the internal height of the condenser body is less than a        corresponding internal height of each of the inlet and outlet        flow distributor tubes;    -   inlet and outlet flow distributor tubes each being provided in        the walls thereof with one or more slots or holes communicating        with the hollow interior of the condenser body so as to provide        a flow path from the inlet flow distributor tube through the        hollow interior of the condenser and into the outlet flow        distributor tube.

The inlet flow distributor tube in use is attached or otherwise in fluidcommunication with the outlet of the evaporation chamber whereas theoutlet flow distributor tube is attached or otherwise in fluidcommunication with the particle detector.

The configuration of the flow distributor tubes, and the positioning ofthe slots or holes, is such as to provide a substantially uniform flowof gas through the condenser body to the particle detector.

The flow distributor tubes may be, for example, of circular crosssection, oval cross section or polygonal (regular or irregular) crosssection. In one embodiment, the flow distributor tubes are of circularcross section.

The internal cross sectional area of each flow distributor tube ispreferably greater than the internal cross sectional area (internalwidth×internal height) of the condenser body. As an example, if thecross-section of the flow distributors is a circle of internal diameterDt and the condenser internal height is Hc and the internal width Wethen πDt²>Hc*Wc. The ratio of πDt²/(Hc*Wc) should typically be more than1.1 or preferably more than 2 or even more preferably the ratio shouldbe more than 3.

As indicated above, fluid communication between the interiors of theflow distributors 102 and 104 and the interior of the rectangularcondenser may be achieved by providing the walls of the flow distributortubes with elongate narrow slots or an array (preferably linear) ofholes that open into the hollow interior of the condenser body.Preferably fluid communication between the flow distributor tubes andthe interior of the condenser body is provided by means of a narrow slotin the wall of each flow distributor tube. By making the slots narrow,the internal diameters of the flow distributor tubes can be reducedbecause the uniformity of the flow in the condenser is governed by theratio of the internal height (Hc) of the condenser to the width (Ws) ofthe slot. In the present context, the width of the slot means thedimension which is in the same direction as the internal height of thecondenser (in contradistinction to the “length” of the slot where thereference to “length” the dimension which is in the same direction asthe internal width of the condenser). Typically, the ratio of Hc/Wcshould be more than 1.1 or preferably more than 2 or even morepreferably the ratio should be more than 3.

In another embodiment, the walls of the flow distributor tubes containholes evenly distributed along the inlet and outlet of the rectangularcondenser 103 instead of slots. The number of holes Nh should be morethan 1 or preferably more than 4 or even more preferably more than 10.The diameter of the holes Dh should be sufficiently small and can beevaluated from the expression: πDt²>Nh*πDh². The ratio of πDt²/(Nh*πDh²)should be more than 1.1 or preferably more than 2 or even morepreferably the ratio should be more than 3.

The flow distributor tubes can be made from any of a variety ofmaterials. For example, they can be made from stainless steel tube,PTFE, aluminium, or any suitable metal, glass, ceramic or plasticsmaterial. The condenser body is preferably made from a heat conductingmaterial such as a metal, e.g. a steel such as a stainless steel.

One potential problem with the condensers, especially when they have asmall internal cross sectional area or width (e.g. less than 2 mm), isthat condensation on the walls of the condenser can lead to blockage. Inorder to overcome this problem, means may be provided for removingcondensed substance from the interior walls of the condenser. Forexample, the condenser may have one or more drainage ducts extendingalong all or part of its length, the drainage ducts being separated fromthe interior of the condenser by a semi-permeable wall or membranethrough which the liquid condensate can pass, the drainage ducts havingone or more outlets connectable to a pump to extract liquid condensatefrom the ducts. The semi-permeable membranes are constantly filled withthe working fluid and therefore the gas flow cannot penetrate throughthem. By means of such an arrangement, when the vaporisable substancecondenses on the inner wall of the condenser, rather than accumulatingin and blocking the condenser, it is extracted through thesemi-permeable wall into the drainage ducts and away from the condenserinterior. Once extracted, the condensate can either be sent to a wastestorage compartment for later disposal or recycled back to theevaporation chamber.

The ducts can be formed by partitioning the interior of the condenserover at least part of its length by means of one or more longitudinallyextending semi-permeable walls or membranes. The semi-permeable walls ormembranes may be provided with capillaries that draw condensate from theinterior of the condenser into the drainage ducts. For example, thewalls or membranes can be formed from a ceramic or stainless steelfilter material having a capillary size of <0.5 mm, e.g. 1-10 μm.

The condensation apparatus of the invention is designed to be connectedto a particle detector, typically a particle detector capable of singleparticle detection and/or single particle counting.

More typically, the condensation apparatus of the invention is designedto form part of a Condensation Particle Counter and, for this purpose,can be connected to a particle counter which can be, for example, aNaneum ‘SAC 1’ particle counter available from Naneum Limited ofCanterbury, United Kingdom.

Accordingly, in another aspect, the invention provides a condensationparticle counter comprising a condensation apparatus of the invention asdefined herein.

In another aspect, the invention provides a method of detecting andcounting nano-particles using a condensation particle counter comprisinga condensation apparatus of the invention as defined herein.

Further aspects and features of the invention will be apparent from thespecific embodiments described below and illustrated in FIGS. 2 to 12.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side sectional view of a known type ofcondensation apparatus.

FIG. 2 is a schematic view of a condensation apparatus according to afirst embodiment of the invention.

FIG. 3 is a schematic view of a condensation apparatus according to asecond embodiment of the invention.

FIG. 4 is a schematic view of a condensation apparatus according to athird embodiment of the invention.

FIG. 5 is a schematic view of a condensation apparatus according to afourth embodiment of the invention.

FIG. 6 is a schematic side sectional elevation of a rectangular crosssection condenser/outlet provided with working fluid removal means thatcan be substituted for the condenser/outlets of any one of theembodiments of FIGS. 2 to 5.

FIG. 7 is a schematic side sectional elevation of an alternativerectangular cross section condenser/outlet provided with working fluidremoval means that can be substituted for the condenser/outlets of anyone of the embodiments of FIGS. 2 to 5.

FIG. 8 is a schematic view of a condensation apparatus according to afifth embodiment of the invention.

FIG. 9 shows a graph comparing of aerosol particle number concentrations(N) measured with the condensation chamber of FIG. 2 coupled with aMetOne™ laser optical particle counter (black squares) and handheld 3007CPC from TSI (white diamonds).

FIG. 10 is a schematic view of a condensation apparatus according to asixth embodiment of the invention.

FIG. 11 is a schematic view of a condensation apparatus according to aseventh embodiment of the invention.

FIG. 12 is a schematic representation of a condensation apparatus andparticle counter assembly according to an eighth embodiment of theinvention, wherein the assembly has the ability to recycle workingfluids.

FIG. 13 is a schematic view of the rectangular condenser with a flowdistributor in the entrance to the condenser and in the outlet of thecondenser.

FIG. 14 is a cross-sectional view (vertical plane) of the rectangularcondenser with a flow distributor in the entrance to the condenser andin the outlet of the condenser.

FIG. 15 is a cross-sectional view (horizontal plane) of the rectangularcondenser with a flow distributor in the entrance to the condenser andin the outlet of the condenser.

FIG. 16 is a cross-sectional view (vertical plane) of the rectangularcondenser with a flow distributor containing a slot in the entrance tothe condenser and another flow distributor with a slot in the outlet ofthe condenser.

FIG. 17 is a sectional view through an evaporation chamber according toanother embodiment of the invention.

FIG. 18 is a schematic sectional elevation showing the evaporationchamber of FIG. 17 from another angle but with the heating elementomitted and a working fluid reservoir in place.

FIG. 19 is a schematic sectional elevation showing a plan view of theapparatus depicted in FIG. 18.

FIG. 20 is a more detailed sectional elevation of the heating elementand porous support used in the embodiment of FIGS. 17 to 19.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be illustrated in greater detail by reference tothe specific embodiments described in the following non-limitingexamples.

FIG. 1 is a schematic side sectional view of a known type ofcondensation apparatus that can be used in combination with a particlecounter. One such known condensation apparatus is the apparatus found inthe handheld CPC 30007 from TSI (www.tsi.com). The apparatus comprisesan evaporation chamber 1 a with a porous support in the form of acartridge 2 a soaked in a volatile working fluid. The chamber 1 a has acylindrical shape and an internal diameter that is greater than theexternal diameter of the cartridge 2 a. The evaporation chamber 1 a isprovided with an inlet 4 a and outlet 5 a, and a heating element 3 a,which encircles the outer wall of the chamber. The temperature of thechamber is measured by a sensor 6 a and controlled by a control unit 7 ausing interface 8 a connected to the heating element on the outer wallof the chamber. A condenser 9 a is placed between the chamber 1 a andthe outlet 5 a.

A stream of air containing small gas-entrained particles (e.g. airborneparticles) is drawn into the chamber 1 a via inlet 4 a by means of apump (not shown). As it passes through the evaporation chamber, thestream of air is heated and saturated with vapour formed by evaporationof the working fluid. The vapour-saturated stream of air then passesinto the condenser where cooling of the air and condensation of theworking fluid onto airborne particles takes place. As a result, theparticles grow by condensation up to a readily detectable size of about1 μm. The enlarged particles pass out through the outlet 5 a, and aredirected to an optical particle counter where they are counted.

The condensation apparatus illustrated in FIG. 1 can be used to detectand count particles in the size range from 10 nm to 600 nm. However, theapparatus suffers from a number of disadvantages.

One major disadvantage is that the working fluid must be replaced on aregular and frequent basis.

A further disadvantage is that the apparatus is very slow to warm up toan operating state. In the case of the TSI CPC 3007 described above, theapparatus has a 600 second warming up time before it can be used.

Another disadvantage is that the layout of the apparatus does notreadily lend itself to miniaturisation. Reducing the size of theapparatus would necessitate using a smaller working fluid cartridgewhich would therefore need to be refilled more frequently. Thus,miniaturisation would lead to a reduction in the period of time overwhich the apparatus could be used without refilling.

A further disadvantage is that the abovementioned TSI CPC 3007instrument cannot be used in an environment of elevated pressure and,according to its product specification, the instrument will only operatewhen held horizontally.

The relatively rapid depletion of working fluid in the cartridge in theCPC shown in FIG. 1 affects the performance of the condensation unitbecause it leads to a lower concentration of vapour of the working fluidin the air inside the chamber. When the vapour concentration is lower,vapour tends to condense preferentially on relatively large particlesand therefore the smaller particles are not detected and counted. Forinstance, the lower detection limit can increase by as much as 10 nm to15 or 20 nm due to working fluid depletion. Since it is very difficultto monitor the extent or rate of depletion in practice, there is achance that many small particles will not be detected and counted.

The condensation apparatus of the invention overcomes or at leastalleviates the problems identified above with known CPCs.

A condensation apparatus according to a first embodiment of theinvention is shown in FIG. 2. The apparatus comprises:

-   -   An evaporation chamber 2 with one inlet 1 (the “first inlet”)        for a clean gaseous medium, e.g. a clean air where all relevant        aerosol particles have been removed.    -   Another inlet 8 (the “second inlet”) through which a stream of a        sample gas (e.g. air) containing nano-particles of interest can        be drawn into the evaporation chamber. The inlet 8 is positioned        at a distance from the first inlet, e.g. at the top of the        chamber while the first inlet 1 is positioned at the bottom of        the chamber.    -   A heating element 3 which is in sufficiently close thermal        contact with a temperature sensor 4, the element being covered        by a porous support 6 wettable by the working fluid. The porous        support 6 is soaked with a working fluid of low equilibrium        vapour pressure, for instance a semi-volatile organic compound        such as dimethyl phthalate.    -   A temperature controlling device 5 that keeps the temperature of        the surface of the porous support 6 at a level sustaining        formation of a sufficient high vapour density of the working        fluid;    -   A condenser/outlet 7 made of a material of high thermal        conductivity and sufficiently long to let droplets of the        working fluid be formed on nano-particles of interest and to        grow to a size that enables them to be detected and counted by        means of single particle counting. The condenser/outlet 7 is        positioned near or opposite the second inlet 8 to provide        effective mixing of the carrier gas and sample gas streams        entering the chamber through the first and the second inlets.        The condenser/outlet 7 is positioned in line with the second        inlet.    -   A temperature cooling device 10 that keeps the temperature of        the surface of the condenser/outlet 7 at a level sustaining        formation and growth of the working fluid droplets. The cooling        device 10 is placed in a position that enables effective cooling        of the condenser/outlet 7 to be sustained.

In use, a stream of clean carrier gas (e.g. air) which has been filteredthrough a filter (not shown) and which contains no (or negligiblequantities of) detectable aerosol particles enters through inlet 1 intothe evaporation chamber 2. In the evaporation chamber, heating element 3is positioned to be in a good thermal contact with temperature sensor 4and a predetermined temperature that is sufficient to evaporate workingfluid and to generate conditions necessary to sustain condensation ofthe working fluid on particles of interest is controlled by temperaturecontrol device 5 which is linked to the temperature sensor. The workingfluid is contained in a porous support that takes the form of a cover 6that is placed on the heating element 3. In the case of a cylindricalheating element, the cover will be wrapped around its surface and soakedwith the working fluid. As a result, the stream of clean air introducedvia the inlet 1 is saturated with the vapour of the working fluid andmoved towards the condenser/outlet 7. A gas (e.g. air) sample containingnano-particles of interest is introduced into the chamber through thesecond inlet 8. In the zone between the inlet 8 and the opening into thecondenser 7, supersaturation of the working fluid vapours arises due tomixing of the hot saturated vapour and the unheated stream of sample gascontaining the nano-particles of interest. Therefore, in this zone,heterogeneous nucleation of the working fluid on the particles ofinterest begins to occur. When the mixture of vapour and sample gascontaining the nano-particles enters the condenser 7, additionalsupersaturation occurs due to cooling of the gases and vapour by thewalls of the condenser/outlet. The excess of heat is removed from thesurface of the condenser/outlet 7 by means of a cooling system 10. InFIG. 2 a flow of cold air 9 generated by a blower is shown. In thecondenser/outlet droplets of the working fluid grow on thenano-particles up to detectable sizes for instance 1 μm. These dropletscan be counted for instance using an optical particle counter (notshown) that is connected to the exit of the chamber 11. Therefore everynano-particle of interest is detected individually.

As an example of the apparatus of shown in FIG. 2, a miniaturecylindrical condensation chamber of 10 mm ID was built from PTFE withinlets 1 and 8 made from stainless steel tube of 4 mm OD and acondenser/outlet 7 made from 8 mm OD stainless steel tube. A K-typethermocouple was utilised as a temperature sensor 4. The heater 3 usedwas a NiCr heater covered by a layer of porous SiO₂ soaked with dimethylphthalate. Particles were counted using a MetOne laser particle counter(Hach ULTRA Analytics).

The temperature was controlled by a Digitron Temperature controller 5.It was found that the condensation apparatus thus constructed was ableto enlarge nano-particles up to 1 to 2 μm diameter. The condensationapparatus was used over a period of at least 2 months without refilling.

Another embodiment of the present invention is shown in FIG. 3. Theapparatus of FIG. 3 is similar to the apparatus of FIG. 2 except that areservoir 12 of working fluid is provided to extend the period of timeover which the apparatus can be operated without refilling. In order toprevent the fluid from the reservoir 12 from escaping and interferingwith the condensation process when the apparatus is not in a horizontalposition, the inlets 1 and 8 and the condenser/outlet 7 are arranged sothat they extend inwardly into the condensation chamber 2. Theextensions prevent the escape of working fluid through the inlets 1 and8 and outlet 7. A condensation apparatus of the type depicted in FIG. 3has been shown to work for at least 12 months without refilling.

The mode of action of this embodiment is the same as for the embodimentof FIG. 2 above.

A further embodiment of the invention is illustrated in FIG. 4. In thisembodiment, which is similar in construction to the apparatus of FIG. 2,a cooling element 13 is provided which is in thermal contact with thecondenser/outlet. The cooling element 13, which can be, for example, athermoelectric cooling element, is attached to the surface of thecondenser/outlet. The cooling element 13 enables heat to be removed fromthe condenser/outlet in a more efficient manner than is possible with afan. The enhanced cooling effect of the cooling element 13 increasessupersaturation of the vapour within the condenser and enables dropletsof the working fluid to grow more rapidly. The operation of the coolingelement 13 can be controlled by so that the temperature of the surfaceof condenser/outlet 7 is lower than the ambient temperature, andtherefore the apparatus can be used effectively over a range of ambienttemperatures including hot environments. The performance of thisembodiment is not influenced by ambient temperature variations.

The shape of the condenser/outlet 7 can affect its performance. In theembodiment shown in FIG. 5, the condenser/outlet 7 has a rectangularcross-section, illustrated by the element labelled as 14. In thisembodiment, the cross-section 14 refers to the middle region of thecondenser/outlet 7. To either side of the middle region, the condensercan be of any cross sectional shape, e.g. circular. The rectangularcross sectional shape presents a greater surface area onto which thestream of cooling air 9 from the fan 10 can be directed therebyenhancing the efficiency of the cooling of the condenser/outlet 7 whichin turn assists the supersaturation of the vapour in thecondenser/outlet 7 and improves the efficiency of droplet growth.Although the condenser/outlet 7 is shown as having a middle region of aflat sided rectangular cross section, the shorter sides of the rectanglecan be rounded instead of flat so that the cross section is in the shapeof an elongated oval rather than a regular rectangle. An elongated ovalcross section can be created by the simple expedient of flattening aportion of the tube from which the condenser/outlet 7 is made.

The second inlet 8 can also have a rectangular or elongated oval crosssectional shape. In practice, the ratio of the height to the width ofthe rectangle can be from 1 to 100.

In each of the embodiments shown in FIGS. 2 to 5, the second inlet 8 ispositioned in line with the condenser/outlet and the cross sectionalarea of the interior of the inlet 8 is less than the cross sectionalarea of the interior of the condenser/outlet 7. Without wishing to bebound by any theory, it is believed that this arrangement results in thestream of sample gas (e.g. air) containing the aerosol particles beinginjected into the centre of a stream of vapour and carrier gas so thatthe mixture entering the condenser/outlet 7 outlet consists of a corestream of sample gas containing the nano-particles surrounded by asheath of carrier gas and vapour. Mixing between the concentric layerstakes place as gases and vapours move along the condenser/outlet 7.

The cross sectional shape of the main body of the condensation chamber 2can also be rectangular as can the cross sectional shape of the heatingelement 3. The heating element 3 is typically positioned and orientatedso as to optimise the efficiency with which the stream of carrier gasentering the first inlet 1 is saturated with the working fluid vapour.

If the width of the rectangular cross section condenser shown in FIG. 5is small (e.g. less than 2 mm), then there is a risk that condensedworking fluid will build up on the interior surface of the condenserthereby clogging the condenser. In order to prevent this from happening,means can be provided for removing condensed working fluid form thecondenser. One way of doing this is to remove the condensed liquid fromthe internal surfaces of the condenser by means of a combination of acapillary action and a pressure differential. An arrangement foraccomplishing this is shown in FIG. 6.

FIG. 6 is a cross sectional view of the condenser. The interior of thecondenser is provided with two solid porous membranes 15 which arewettable by the working fluid and serve to partition the interior of thecondenser into a central passage and a pair of elongate fluid collectionchambers 16. In use, as the stream of vapour and gases containing thenano-particles and growing droplets passes along the central passagebetween the membranes 15, condensation of working fluid vapour onto thesurface of the membranes takes place. The excess liquid thus formed isimmediately sucked through the membranes 15 and into the collectionchambers 16 by the combination of capillary action and a negativepressure maintained by a pump (not shown). From the chambers 16, theliquid is removed through the outlets 17 and can then be directed to awaste collection chamber (not shown) or, in the case of an apparatushaving a working fluid reservoir 12 as shown in FIG. 3, recycled to thereservoir.

It is advantageous to control the temperature of the liquid in theworking fluid containers 16 to stabilise the temperature of thecondenser. This can be done using an external cooling element (e.g.thermoelectric cooling element) as shown in FIG. 4 or can be achieved bycirculating the liquid through a heat exchanger as shown in FIG. 7.

FIG. 7 is a schematic illustration of a cross-section of part of acondenser (the rest of the condenser is not shown) provided with fluidcollection chambers 16 which are connected to a working fluid recyclingand temperature control circuit.

The chambers 16 each have an additional outlet 18 and the two outletsare connected by a length of tubing. The inlets/outlets 17 are connectedvia lengths of tubing to a pump 19 and a temperature controller 20.Together, the inlets/outlets 17 and 18, the connecting tubing, the pump19 and the temperature controller 20 form a circuit around which theworking fluid can be pumped. The working fluid flowing around thecircuit and through the fluid collection chambers 16 can be maintainedat a constant pre-determined temperature by the temperature controller20 and, in this way, the temperature of the internal surface of thecondenser can be controlled.

The circuit is provided with a valve (not shown) that enables a portionof the working fluid to be directed along tube 21 to a reservoir 12 ofworking fluid in the main body of the evaporation chamber 2 by means ofan additional pump (not shown) or other liquid transporting means.

The tubing connections shown in FIG. 7 are merely illustrative and itwill be appreciated that the connections can be arranged differently toenable the liquid to cool the condenser more uniformly. For example, inan alternative arrangement, the working fluid can be directed from thecontroller 20 to the inlets 17 and removed from outlets 18 to the pump19.

In addition, it should be appreciated that the left and the right fluidcollection chambers 16 can be maintained at different temperatures. Thisgenerates extra supersaturation of the vapours of the working fluid inthe condenser and enables the rate of growth of the droplets and thedroplet size to be increased or decreased as required. In this case eachchamber has its own temperature controlling cycle. The temperatures ofliquid in the containers can be found experimentally by means of trialand error or calculated according to nucleation theory.

Keeping two fluid collection chambers 16 at different temperatures hasanother important advantage. When supersaturation in the condenser issufficiently high, nano-particles of different sizes can form dropletsin different locations along their travel through the condenser and,therefore, droplets formed onto nano-particles of different sizes willgrow to various sizes. For instance, 50 nm particles will produce 0.5 μmdroplets but 100 nm particles will generate 1 μm droplets. This enablesthe size of the nano-particles to be obtained from the size of thedroplets, a facility which can form the basis for methods ofcharacterising aerosol size distributions.

It should be also appreciated that the temperature of the internalsurface of the condenser can be non-uniform, for instance, it canlinearly decrease with the length of the condenser. This graduallyincreases supersaturation of the working fluid vapour along the lengthof the condenser and, therefore, increases the ability of the apparatusto grow nano-particles of different sizes up to droplets of differentsizes. Larger nano-particles tend to form droplets earlier (at thebeginning of the condenser) whereas smaller particles that requiregreater supersaturation tend to form droplets only later at the end ofthe condenser and consequently the smaller particles have less time togrow and therefore grow to smaller droplet sizes in comparison withlarger nano-particles. This makes it possible to establish a one-to-onerelationship between the size of droplets formed in the condensationchamber and the size of nano-particles. This relationship can beutilised to evaluate the size of nano-particles by analysing the size ofthe droplets.

Supersaturation in the apparatus of the invention is controlled by thetemperature of the walls, the dimensions of the component parts of theapparatus and the flow rates of the carrier gas and sample gas streamsthrough the apparatus. Variation of these parameters enables a skilledperson to select the supersaturation conditions. There is a well-knownlink between the supersaturation and the minimal size of nano-particlesthat can form droplets. Therefore, it is possible to change the lowerdetection limit of a condensation apparatus by changing one or severalof these parameters, e.g. the temperature of a heating element 3. Thisis a powerful tool in determining size distributions of nano-particlesand the proportion of nano-particles in various size ranges. It alsoenables the development of a condensation apparatus with a predeterminedlower detection limit, e.g. 100 nm, 30 nm, 10 nm or 3 nm or with avariable lower detection limit. This provides a platform for an aerosolparticle sizing in order to obtain nano-particle size distributions.

It also should be appreciated that a plurality of condensationapparatuses of the invention set up to give different supersaturationconditions can be connected to each other sequentially or in parallel.The sequential arrangement enables nano-particles of different sizes togrow up to different size droplets. If the first condensation chamber isset at lower supersaturation than the second then larger particles formdroplets in the first chamber but smaller particles form droplets onlyin the second chamber whereas previously formed droplets are grownfurther and become distinctly larger in size. The same is true for thesecond and the third chambers. Thus, a plurality of chambers enables aplurality of droplet sizes to be formed. This allows the sizedistribution of nano-particles to be retrieved by analysing the sizedistribution of droplets, e.g. by using an optical particle counter.

In the case of a parallel arrangement of condensation chambers, thestream of nano-particles of interest is divided into several parallelflows and the said flows are directed to different chambers. Thechambers should be set to different values of supersaturation so as tohave different lower size detection limits. This makes it possible toretrieve nano-particle number size distributions by analysing thenumbers of droplets grown in these chambers.

It will also be appreciated that it is possible to vary temperatures andother parameters of the condensation apparatus and therefore vary thesupersaturation as well as the lower detection limit during a givenmeasuring cycle. This enables a cumulative particle size distribution tobe obtained.

An apparatus according to a further embodiment of the invention isillustrated schematically in FIG. 8. In this embodiment, the apparatuscomprises an evaporation chamber (saturating chamber) 2 containing aheating element 3 powered by a controlled power supply 5. The heatingelement 3 is in close contact with a porous material 6 soaked with aworking fluid, e.g. semi-volatile compound, attached to a temperaturesensor 4 connected to the power supply 5. The evaporation chamber has aninlet 1 and an outlet which extends into a nozzle 22 opening into thecondenser 7. The condenser is provided with an inlet 8 through which agas sample containing nano-particles of interest can be introduced intothe condenser. The nozzle 22 extends into the condenser so that it opensout into the condenser downstream of the inlet 8. The condenser 7 isequipped with an outlet 11 and a cooling element 13.

The embodiment of FIG. 8 works as follows. A stream of the clean air(carrier gas) is directed into to the saturating chamber 2 via inlet 1by means of a flow-generating device, e.g. a pump (not shown). In thechamber, the heating element 3 heats the porous material 6 soaked in theworking fluid to produce vapour. The air stream containing the vapour isdirected to the condenser through nozzle 22 where the air stream iscooled upon mixing with the unheated stream of sample gas containingnano-particles entering through inlet 8. The nozzle 22 is designed todeliver the hot vapour-saturated air stream into the centre of thesample gas stream containing nano-particles so that the hot vapoursaturated stream is surrounded by a sheath of cooler sample gas. Thecombined gas streams are cooled by the cooling element 13 that controlsthe temperature of the walls of the condensing chamber 7. In thecondenser, supersaturation of the working fluid occurs as a result ofthe mixing of the hot vapour-saturated air with the cooler sample gasand cooling by the walls of the condenser 7. Thus leads to condensationof the working fluid vapour onto the airborne nano-particles and theformation of droplets of about 1 μm. These droplets are directed to anoptical particle counter via outlet 11 and counted individually.

In order to reduce particle losses, both the nozzle 22 and the condenser7 have cylindrical symmetry and the nozzle 22 is positioned along theaxis of the condenser 7 in such a way that the end of the nozzle extendsdownstream beyond the second inlet 8. This enables the cooler sample gasstream to be formed around the vapour-containing carrier gas stream.

An advantage of the condensation apparatus of the invention is that itprovides reliable data and can be miniaturised to dimensions muchsmaller than those of known condensation counters

A preferred working fluid in each of the embodiments of the invention isthe semi-volatile dimethyl phthalate. A major advantage of using asemi-volatile compound is that it leads to a much lower consumption ofworking fluid. An apparatus of the invention has been found to workwithout requiring refilling for more than 10 months.

The choice of flow rates, the temperature of the saturating chamber andthe manner in which airborne particles of interest are introduced intothe chamber will usually be made according to the nature of theparticles and their concentration. The total flow out of the condenseroutlet 11 is often in the range from 0.1 to 4 l/min. The clean carriergas flow accounts for 10 to 90% of the total flow. For dimethylphthalate, the temperature of the saturating chamber usually is in therange from 80 to 150° C.

In order to reduce the power consumption of the heating element, a thinfilm heater can be used which has attached to it a porous medium whichis wettable by the working fluid. It is advantageous for a part of theporous medium to be long enough to be in contact with working fluid atthe bottom of the chamber 2.

The evaporation chamber 2 and condenser 7 may be manufactured from avariety of materials including any metal, glass or ceramic or (in thecase of the evaporation chamber) plastics such as PTFE, but it ispreferred to use materials or surface treatments that are inert orresistant to oxidation in air or other carrier gases and which do notreact chemically with the working fluid. Pyrex glass, quartz, ceramicand stainless steel were used for various modifications of the chambersand their elements.

It will also be appreciated that the sample gas stream containingparticles of interest can be introduced through inlet 1 and the cleanair via inlet 8. This is preferable for temperature stable particlessuch as metal particles. However, aerosol particles formed from organiccompounds can be affected the high temperatures generated by the heatingelement and should therefore be introduced via inlet 8.

An apparatus according to another embodiment of the invention isillustrated in FIG. 10. In this embodiment, in which the evaporationchamber has a similar layout as the embodiment of FIG. 8, a mixingchamber (intermediate chamber) 25 is positioned between the saturatingchamber 2 and the condenser 7. The mixing chamber is divided by apartition 24 having a central orifice into a downstream sub-chamber andan upstream sub-chamber. The nozzle 22 extends into the upstreamsub-chamber and a cylindrical baffle, which is aligned with the nozzle22 extends in an upstream direction from the end of the downstreamsub-chamber. These elements can be made from the same materials as therest of the apparatus. The mixing chamber 25 and partition 24 can havecylindrical symmetry or they can be rectangular in cross section. Thisembodiment enables to achieve the lowest low detection size ofnano-particles of 1 nm. A third inlet 23 is provided and this opens outinto the annular space surrounding the cylindrical baffle.

In this embodiment, a sample of aerosol of interest is directed intoinlet 8 and a stream of clean air is introduced through inlet 23. Themixing chamber enables the stream of the aerosol sample of interest tobe sandwiched between a central core stream of carrier gas containingworking fluid vapour and a outer layer formed by the clean air frominlet 23. Using an apparatus of this type, it is found that the bestresults are obtained when the gas layers in the sandwich arecylindrically symmetrical.

It is advantageous to prolong the working life of the apparatus withoutthe need for frequent refilling with working fluid. A significantlylonger operating life between refills can be achieved by means of acombination of two condensation units with means for collecting andrecycling working fluid from droplets that have passed though theparticle detector and which contain airborne particles. Such an assemblycan comprise two condensing units and a system of aerosol flowmanipulation with three way valves to redirect the flows. Thecondensation apparatus used with such an assembly is slightly differentfrom the other specific embodiments described above and an example of asuitable condensation apparatus is shown in FIG. 11. In the embodimentof FIG. 11, the inlet 1 receives gas (e.g. air) in which is suspendedmicro-droplets containing airborne particles recycled from the particlecounter. In order prevent the recycled airborne particles fromcontinuing through to the condenser and hence contaminating the samplegas containing the particles of interest, the porous medium 26 ispositioned across the evaporation chamber airflow in the evaporationchamber. The porous medium is selected so as to be capable of performingtwo functions. Firstly, it must be wettable by the working fluid sothat, when heated, it can serve as source of vapour and, secondly, itmust be capable of functioning as a filter to collect micro-dropletscontaining microparticles thereby avoiding contamination of the vapourin the region downstream of the porous medium. As in the otherembodiments described above, a heating element 3 is positioned near thesurface of the porous medium to evaporate deposited working fluid.However, thermal contact between the heating element 3 and the porousmedium 26 is not important in this embodiment because of the heatingeffect provided by the gas entering the chamber through inlet 1.

It should be understood from the above that the porous medium 26 shouldbe positioned in such a way to form an airtight seal with the walls ofthe evaporation chamber so that all of the gas received through inlet 1is filtered and all of the airborne particles are trapped. Droplets ofworking fluid collected on the porous support can be re-evaporated andreleased as vapour into the evaporation chamber on the downstream sideof the porous support

An assembly comprising two condensation apparatuses of theaforementioned type consumes negligible or no working fluid andtherefore does not need to be refilled. Such an assembly is illustratedin FIG. 12.

The assembly shown in FIG. 12 comprises two apparatus 27 and 28 eachcorresponding to the apparatus illustrated in FIG. 11 and an opticalparticle counter 29. Three three-way valves 30, 31 and 32 and two on/offvalves 33 and 34 direct the flows of fluid to permit recycling of theworking fluid.

The assembly shown in FIG. 12 functions as follows:

Airflow containing nano-particles of interest is drawn via common inlet35 into one condensation apparatus (e.g. apparatus 28) by appropriateadjustment of the three-way valve 30. Micro-droplets formed on thenano-particles in the apparatus are directed towards optical particlecounter 29 by means of three-way valve 31 while valve 34 is closed.After being counted, micro-droplets are directed to the other apparatus27 by three-way valve 32. In apparatus 27, micro-droplets are collectedonto the porous medium and filtered clean air is released into theatmosphere through outlet 35 by opening valve 33. The evaporationchamber in apparatus 27 remains cold because there no voltage is appliedto the heating element, and therefore the working fluid collected on theporous medium is stored.

After an appropriate period of time, the apparatus 27 is heated, thevalve positions are adjusted and apparatus 28 is allowed to cool so thatit is able to collect droplets of working fluid. The previously idleapparatus 27 is then in working mode and the working fluid previouslycaptured by the porous medium is heated and evaporated to form a vapourwhich is then mixed with a gas sample stream as described above. Afterpassing through the condenser and particle counter, the stream ofparticle- and vapour-laden air is directed to apparatus 28 where the airis filtered and the working fluid collected as described above forapparatus 27. The cycle is then repeated.

The time necessary to switch between the chambers can be determinedempirically through trial and error. Normally it is only necessary toswitch the valves after hundreds of hours of operations. Therefore, thesystem requires relatively little energy to operate and energy and canbe easily implemented.

If necessary, additional specialised gas filters can be attached tooutlets 35 and 36 to trap working fluid vapour remaining in the gasstream after filtration by the porous support. However, by using asemi-volatile working fluid such as dimethyl phthalate, for the majorityof applications there is no need to use additional filters because thevapour pressure of the semi-volatile compound is very low.

In each of the foregoing embodiments, the condensation chamber can beequipped with a working fluid sensor, e.g. filled glass capillary or dewpoint type sensors (not shown in the Figures). A sensor placed e.g.inside the chamber enables the depletion of the working fluid to bemonitored.

FIGS. 13 to 16 illustrate a further type of condenser for use in theapparatus of the invention. In this embodiment, the condenser is ofrectangular cross section.

By using a condenser of rectangular cross-section, the size of thecondensation chamber can be reduced considerably. However, a potentialproblem with some rectangular condenser layouts, particularly where theinlet and outlet of the condenser are tubes of circular cross section,is that there may be non-uniformity of the flow velocity in thecondenser. This can lead to some particles spending more time in thecondenser than others meaning that there is non-uniform growth of theparticles in the condenser. This in turn can give rise to inaccuraciesin measurement of the numbers and sizes of the particles. FIGS. 13 to 16illustrate a rectangular condenser which provides a substantiallyuniform flow velocity of the vapour laden gases through the condenser.In this embodiment, a pair of flow distributors is provided, oneattached either side of the rectangular condenser. One of the flowdistributors is in fluid communication with (e.g. connected to) theevaporation chamber, and the other flow distributor is in fluidcommunication with (e.g. is connected to) a particle detector.

Thus, as shown in FIG. 13, a rectangular condenser 103 is equipped withthe first flow distributor 102 and a second flow distributor 104, bothof which are in fluid communication with the condenser. The hot gas(e.g. air) saturated with working fluid enters the first flowdistributor 102 via an inlet 101. The flow distributor 101 is designedto supply a uniform flow to the condenser 102. Therefore, the oppositeto the inlet 101 end of the flow distributor 102 is blocked, see FIG.15. At the outlet of the condenser 103, the second flow distributor isattached to reduce non-uniformity of the flow at the outlet of thecondenser 103. The gas flow leaves the second flow distributor throughan outlet 105.

FIG. 14 shows the relative positions of the first flow distributor 101,the rectangular condenser 103 and the second flow distributor 104.

FIG. 15 shows schematically the flow stream lines inside the condenser103, the flow distributor 102 and the flow distributor 104. In thisembodiment, the air parcels moving along trajectories 106, 107 and 108have substantially the same velocity and therefore substantially thesame residence time in the rectangular condenser 103 and as a resultsubstantially the same sizes of droplets formed.

The uniformity of the residence time is achieved by designing the flowdistributors such that the internal area of the cross-sections of theflow distributors are sufficiently greater than the internal area of thecross-section of the condenser. As an example, if the cross-section ofthe flow distributors is a circle of the internal diameter Dt and thecondenser internal height is Hc and the internal width We thenπDt²>Hc*Wc. The ratio of πDt²/(Hc*Wc) should be more than 1.1 orpreferably more than 2 or even more preferably the ratio should be morethan 3.

Fluid communication between the interiors of the flow distributors 102and 104 and the interior of the rectangular condenser can be achieved byproviding the walls of the flow distributors with elongate narrow slotsor a linear array of holes that open into the condenser. It isadvantageous to provide a narrow slot between the flow distributors 102and 104 and the condenser 103. In FIG. 16, the vertical cross-sectionalview shows schematically the slots 109 and 110. In this case theinternal diameter of flow distributors can be reduced because of theuniformity of the flow in the condenser is governed by the expressioncontaining the width of the slot Ws but not the height of the condenserHc:Hc>Ws. The ratio of Hc/Wc should be more than 1.1 or preferably morethan 2 or even more preferably the ratio should be more than 3.

In another embodiment, the flow distributor contains holes evenlydistributed along the inlet and outlet of the rectangular condenser 103instead of two slots. The number of holes Nh should be more than 1 orpreferably more than 4 or even more preferably more than 10. Thediameter of the holes Dh should be sufficiently small and can beevaluated from the expression: πDt²>Nh*πDh². The ratio of πDt²/(Nh*πDh²)should be more than 1.1 or preferably more than 2 or even morepreferably the ratio should be more than 3.

In one embodiment of the condenser arrangement shown in FIGS. 13 to 16,the flow distributors 102 and 104 were manufactured from a stainlesssteel tube of 7 mm ID and the length of 40 mm. The condenser 103 wasmanufactured from stainless steel sheet with dimensions: Wc=20 mm,Hc=1.4 mm and Lc=30 mm. However, it should be appreciated that othermaterials can be used to manufacture the flow distributors, e.g. PTFE,aluminium, or any suitable metal, glass, ceramic or plastics material.

It will also be appreciated that the shape of the cross-section of thedistributor may be rectangular, triangular, ellipsoidal, polygonal orany combination of simple geometric shapes.

FIGS. 17 to 19 illustrate an evaporation chamber and associated workingfluid reservoir.

The evaporation chamber shown in FIG. 17 comprises a body 200 formedfrom PTFE enclosing a chamber interior 216. The chamber has a pair ofinlets 220 (not shown in FIG. 17 but see FIG. 18) and has means 202 forconnection to a condenser (not shown).

A heating element is mounted in one side of the PTFE body 200. Theheating element has a mounting portion 206 removably secured in the wallof the PTFE body, and a rod portion 210 which extends into the chamberinterior 216. A holder 204 holds the mounting portion 206 in place andan O-ring provides a seal between the mounting portion 206 and the wallof the chamber body. Another O-ring provides a seal between the mountingportion 206 and the rod 210.

The rod portion 212 has a hollow interior within which are disposed ametal heating wire 222 and a thermocouple 224 (see FIG. 20). The heatingwire 222 and thermocouple 224 are secured in place by means of a thermalfiller (222) which can be, for example, a solder or a metal filled epoxyresin.

The heating wire and thermocouple are connected to a controller (notshown).

Beneath the heating element, the body of the evaporation chamber has awell 214/218 for holding a working liquid such as dimethyl 1phthalate.The well 214/218 is connected via tube 218 (see FIG. 18) to a reservoir217 of the working fluid.

As shown in FIG. 20, the rod portion is surrounded by a sleeve 226 of aporous material which, in this embodiment, is a porous fabric such asquartz fibre filter or glass fibre filter or a polymer or metal filter.The sleeve of porous fabric has a tail portion 228 which extends intothe well and acts as a wick to draw working fluid up from the well. Thesleeve is held in place on the rod portion by means of a clip 230 whichis provided with holes through which the working liquid can evaporateand an opening on its underside to accommodate the tail portion of thesleeve. The wick is surrounded by a wick holder 231 made from an inertmaterial e.g. stainless steel.

In this embodiment, the heating element is in direct contact with theporous support thereby reducing the heat input required and time takento evaporate the working fluid to form a saturated vapour within thechamber.

EXAMPLES

Several examples of apparatus according to this invention have beenbuilt and tested and these are described below.

Example 1

In one example constructed as shown in FIG. 8, the evaporation chamber(saturating chamber) 2 was made of stainless steel tube (12 mm ID) and30 mm length. Stainless steel tube of 3 mm ID was used for the inlets,nozzle and the outlet. The cooling element was constructed from a 5V DCmicro fan positioned at 15 mm from the surface of the condensingchamber. The condenser chamber was made of stainless steel tube 6 mm IDlength 60 mm and the heating element was made a NiCr heating elementcovered with a quartz fibre material that was sufficiently long to bepositioned near to the bottom of the stainless steel cylinder of thesaturating chamber. About 0.5 ml of dimethyl phthalate was poured intothe chamber 2 as the working fluid. Micro droplets formed ontonano-particles were counted with a MetOne laser optical particlecounter. The apparatus was tested against SMPS (TSI), portable SAC sizespectrometer (Naneum) and a handheld instrument 3007 CPC (TSI, model3007). Nanoparticles of chromium oxides and atmospheric aerosols wereused for the tests. In the tests, it was found that the apparatus of theinvention enabled nano-particles to be enlarged up to 1.2 μm in diameterand the lower limit of the detection range was estimated to be 4 nm.

Example 2

A comparison was made between aerosol particle number concentrations (N)measured using an apparatus as illustrated in FIG. 2 coupled to a MetOnelaser optical particle counter and aerosol particle numberconcentrations (N) measured using a handheld 3007 CPC from TSI. Theclean air flow through inlet 1 into the saturating chamber was set at0.3 l/min and the sample gas flow through inlet 8 was set at 0.5 l/min.Nano-particles of chromium oxide and atmospheric aerosols were used forthe comparative tests.

The results are shown in FIG. 9 where the data points for the apparatusof the invention are shown as black squares and the data points for theTSI 3007 CPC instrument are shown as white diamonds. In FIG. 9, D is themean diameter (nm) which was obtained by calibration using a referencemethod.

It was found that the apparatus of the invention enables nano-particlesto be enlarged up to 1.2 μm in diameter. The lower detection limit forthe apparatus of the invention was estimated to be 3 nm. It is clearfrom FIG. 9 that the lower detection limit of the apparatus of theinvention is lower than the low detection limit of CPC 3007 (TSI).

Equivalents

It will readily be apparent that numerous modifications and alterationsmay be made to the specific embodiments of the invention described abovewithout departing from the principles underlying the invention. All suchmodifications and alterations are intended to be embraced by thisapplication.

The invention claimed is:
 1. Apparatus for increasing the size ofgas-entrained particles in order to render the gas-entrained particlesdetectable by a particle detector, the apparatus comprising: anevaporation chamber; a condenser in fluid communication with theevaporation chamber and having an outlet for connection to a particledetector; a heating element and a porous support each of which isdisposed within the evaporation chamber, the porous support carryingthereon a vaporisable substance and the heating element being heatableto vaporise the vaporisable substance to form vapour within theevaporation chamber; wherein the porous support surrounds the heatingelement; a first inlet for admitting a stream of carrier gas into theevaporation chamber to carry vapour through to the condenser; a secondinlet which is downstream of the porous support and through which astream of sample gas containing gas-entrained particles can beintroduced; the apparatus being configured so that condensation of thevaporisable substance onto the gas-entrained particles in the sample gastakes place in the condenser to increase the size of the particles sothat they are capable of being detected by a particle detector. 2.Apparatus according to claim 1 wherein a temperature sensor is disposedwithin the evaporation chamber.
 3. Apparatus according to claim 1wherein the apparatus is configured such that: the second inlet isarranged so that it opens into an intermediate chamber between theevaporation chamber and the condenser; the intermediate chamber isdivided by a dividing wall into upstream and downstream sub-chambers, acentral hole in the wall providing communication between thesub-chambers, whereby the second inlet opens into the upstreamsub-chamber; a third inlet opens into the downstream sub-chamber, thethird inlet being connectable to a supply of filtered gas; a nozzle isprovided that extends from an exit opening of the evaporation chamberinto the condenser to a position in the upstream sub-chamber that islevel with or downstream of the second inlet; the downstream sub-chambercontains a cylindrical baffle that is aligned with the said nozzle andthe central hole in the dividing wall, and the third inlet opens into aspace surrounding the cylindrical baffle.
 4. Apparatus according toclaim 1 wherein cooling means are provided to assist cooling of themixture of gases, vapour and particles in the condenser.
 5. Apparatusaccording to claim 1 wherein the condenser has a surface area to volumeratio which is greater than the surface area to volume ratio of acylinder.
 6. Apparatus according to claim 1 wherein the condenser isprovided with means for removing condensed substance from the interiorwalls of the condenser.
 7. Apparatus according to claim 1 containingmeans for varying the temperature of the porous support and/or thetemperature of the condenser or its parts in order to control the lowerdetectable size limit of particles of interest and, therefore, to obtainnano-particle size distributions.
 8. An apparatus as defined in claim 1,wherein the condenser comprises: a condenser body having a condenserinlet, the outlet, and a hollow interior which has an internal length,an internal width and an internal height; the condenser inlet providingfluid communication with the evaporation chamber; an inlet flowdistributor tube connected to the condenser inlet and extending acrossthe internal width of the condenser body; and an outlet flow distributortube connected to the outlet of the condenser body and extending acrossthe internal width of the condenser body; wherein the internal height ofthe condenser body is less than a corresponding internal height of eachof the inlet and outlet flow distributor tubes; inlet and outlet flowdistributor tubes each being provided in the walls thereof with one ormore slots or holes communicating with the hollow interior of thecondenser body so as to provide a flow path from the inlet flowdistributor tube through the hollow interior of the condenser and intothe outlet flow distributor tube.
 9. A apparatus according to claim 8wherein the internal cross sectional area of each flow distributor tubeis greater than the internal cross sectional area (internalwidth×internal height) of the condenser body.
 10. A condensationparticle counter comprising an apparatus as defined in claim
 1. 11. Anassembly comprising an apparatus as defined in claim 1 connected to aparticle detector.
 12. An assembly comprising a plurality of apparatusesaccording to claim 1 connected sequentially or in parallel. 13.Apparatus according to claim 1 wherein the heating element comprises arod portion and the porous support surrounds the said rod portion. 14.Apparatus according to claim 13 wherein the porous support is formedfrom a porous fabric and comprises a sleeve that fits over the rodportion of the heating element.
 15. Apparatus according to claim 13wherein the rod portion of the heating element has a hollow interiorwithin which is disposed a heater wire or heater probe and optionally athermocouple.
 16. Apparatus according to claim 15 wherein a thermallyconductive filler is used to hold the heater wire or heater probe andthe thermocouple (when present) in place.
 17. Apparatus according toclaim 1 wherein the vaporisable substance is selected from dimethylphthalate, dioctyl phthalate and dimethylsulphoxide.